This invention relates to vapor-phase reaction processes and, in particular, vapor-phase processes for the preparation of aziridine compounds. More particularly, it relates to processes in which vapor-phase reactions, especially vapor-phase reactions for the preparation of aziridine compounds from alkanolamines, can be carried out with industrial advantages.
Conventionally, aziridine compounds have been prepared according to a liquid-phase process, i.e., a process wherein a sulfuric ester of monoethanolamine is treated with a concentrated alkaline solution to form ethyleneimine. However, this process has many disadvantages from an industrial point of view. For example, this process involves high production costs because large amounts of sulfuric acid and alkali are required as secondary raw materials, and this process yields inorganic salts of less utility value as by-products.
In recent years, therefore, a vapor-phase process wherein an aziridine compound is directly prepared by subjecting an alkanolamine to a vapor-phase intramolecular dehydration reaction in the presence of a catalyst (the vapor-phase intramolecular dehydration reaction in the presence of a catalyst may hereinafter be referred to briefly as the xe2x80x9cdehydration reactionxe2x80x9d) is being employed because it requires no secondary raw material and is hence advantageous from the viewpoint of cost. As to the preparation of aziridine compounds by a vapor-phase reaction, a variety of processes have been proposed.
For example, Japanese Patent Laid-Open No. 217659/""92 discloses a process comprising a reaction step for forming an aziridine compound by the dehydration reaction of an alkanolamine, a collection step for collecting the aziridine compound so formed by bringing the reaction mixture containing the aziridine compound into contact with the same alkanolamine as used in the reaction step (i.e., a collector), a purification step for introducing the collector liquid containing the aziridine compound (hereinafter referred to briefly as the xe2x80x9ccollector liquidxe2x80x9d) into a purification column and purifying the aziridine compound, and a recovery step for recovering the alkanolamine by distilling the bottoms withdrawn from the purification column in the presence of water.
Japanese Patent Publication No. 55498/""93 discloses a process wherein the dehydration reaction of an alkanolamine is carried out in the absence of any diluent gas or in the presence of a small amount of a diluent gas. According to this process, an aziridine compound can be obtained by cooling the reaction gas so as to condense it, and introducing the resulting condensate directly to a distillation step, instead of subjecting the reaction gas to a collection step. Alternatively, this process may include a collection step. In this case, it is said that the amount of collector used can be reduced.
Moreover, Japanese Patent Publication No. 88353/""95 discloses a process wherein, after the dehydration reaction of an alkanolamine, the resulting aziridine compound is collected by bringing the reaction gas into contact with a collector comprising an amine compound or a solution thereof. More specifically, this process comprises cooling the reaction gas, introducing the reaction gas into a collection column where the aziridine compound is collected by contact with a collector, and then introducing the collector liquid into a distillation column where it is distilled to obtain the aziridine compound.
As described above, generally known vapor-phase processes for the preparation of aziridine compounds from alkanolamines include one comprising a reaction step for forming an aziridine compound by the dehydration reaction of an alkanolamine, a collection step for collecting the aziridine compound present in the resulting reaction gas, and a purification step for distilling the collector liquid containing the aziridine compound to obtain an aziridine compound product; and one comprising the reaction step and the purification step and requiring no collection step. Moreover, as described in the aforementioned patents, it is common practice to cool the reaction gas resulting from the reaction step by means of a heat exchanger or the like and then feed it to the collection step or the purification step. More specifically, the reaction gas is cooled to xe2x88x9210xc2x0 C. or 100xc2x0 C. and then fed to a collection column in the process of Japanese Patent Laid-Open No. 217659/""92; the reaction gas is cooled to xe2x88x9210xc2x0 C. and then fed to a distillation column in the process of Japanese Patent Publication No. 55498/""93; and the reaction gas is cooled to 110xc2x0 C. and then fed to a collection column in the process of Japanese Patent Publication No. 88353/""95.
Problems to be Solved by the Invention
However, when an aziridine compound is prepared on an industrial scale, or example, by using a reactor (or reaction column), a collection column and a distillation column, tarry matter adheres to the equipment section extending from the outlet of the catalyst bed within the reactor to the collection column, particularly to the inside of a heat exchanger installed before the collection column for the purpose of cooling, or cooling to condense, the reaction gas and to the inner wall of the pipe extending from the heat exchanger to the collection column When such tarry matter adheres to the inside of the heat exchanger and the inner wall of the piping, various problems arise. That is, (1) an increase in pressure loss is caused to increase the amount of energy required for the vaporization of the alkanolamine raw material fed to the reactor; (2) it becomes necessary to shut down the plant and wash off the tarry matter; and (3) the piping may become clogged.
The above-described problems also arise unavoidably when various vapor-phase reactions, such as vapor-phase intramolecular dehydration reactions and vapor-phase intramolecular dealcoholization reactions, are carried out.
An object of the present invention is to solve the above-described problems of the prior art by providing a process for preparing aziridine compounds with industrial advantages by effectively preventing the adhesion of tarry matter to piping and the like.
Another object of the present invention is to provide a process for carrying out various vapor-phase reactions, such as vapor-phase intramolecular dehydration reactions and vapor-phase intramolecular dealcoholization reactions, with industrial advantages by effectively preventing the adhesion of tarry matter to piping and the like.
Means for Solving the Problems
According to investigations conducted by the present inventors, it has been found that the adhesion of tarry matter can be effectively prevented (1) by maintaining at least the piping section extending from the outlet of the reactor to the inlet of the collection column at a temperature which is not lower than 200xc2x0 C., or (2) by bringing the reaction gas emerging from the outlet of the catalyst bed within the reactor into contact with the same collector as used in the collection column, preferably in the neighborhood of the outlet of the catalyst bed. The present invention has been completed on the basis of this finding.
Thus, the present invention provides a process for the preparation of aziridine compounds which comprises the step of introducing an alkanolamine into a reactor and forming an aziridine compound by the vapor-phase intramolecular dehydration reaction of the alkanolamine in the presence of a catalyst, and the step of introducing the reaction gas containing the aziridine compound so formed into a collection column and collecting the aziridine compound by contact with a collector, the process being characterized in that at least the piping section extending from the outlet of the reactor to the inlet of the collection column is maintained at a temperature which is not lower than 200xc2x0 C.
Moreover, the present invention also provides a process for the preparation of aziridine compounds which comprises the step of introducing an alkanolamine into a reactor and forming an aziridine compound by the vapor-phase intramolecular dehydration reaction of the alkanolamine in the presence of a catalyst, and the step of introducing the reaction gas containing the aziridine compound so formed into a collection column and collecting the aziridine compound by contact with a collector, the process being characterized in that the reaction gas is brought into contact with a collector prior to its introduction into the collection column.
Furthermore, the present invention also provides a process for the preparation of aziridine compounds which comprises the step of introducing an alkanolamine into a reactor and forming an aziridine compound by the vapor-phase intramolecular dehydration reaction of the alkanolamine in the presence of a catalyst, the step of cooling the reaction gas containing the aziridine compound so formed by means of a cooler so as to condense the reaction gas, and the step of introducing the resulting condensate into a distillation column and distilling it therein to obtain the aziridine compound, the process being characterized in that the reaction gas is brought into contact with a collector prior to its introduction into the cooler.
In addition, the present invention also provides a vapor-phase reaction process comprising a vapor-phase reaction step and a collection step for collecting a desired product present in the reaction gas, or a vapor-phase reaction process comprising a vapor-phase reaction step, a cooling step for condensing the reaction gas, and a distillation step for distilling the resulting condensate, the process being characterized in that at least the piping section extending from the outlet of the reactor to the inlet of the collection column is maintained at a temperature which is not lower than 200xc2x0 C., or in that the reaction gas is brought into contact with a collector prior to its introduction into the collection column or the cooler.
The reason why the processes of the present invention can effectively prevent the adhesion of tarry matter to piping and the like has not been fully elucidated as yet. However, the present inventors have theorized on the effects of the present invention as described below, though they do not wish to be bound by any theory. (1) As described in the aforementioned patents, the reaction gas contains carbonyl compounds and various amines (e.g., piperazine) as by-products. It is known that, among these by-products, carbonyl compounds react with the desired product (i.e., the aziridine compound) to form adducts. In conventional processes in which the reaction gas is cooled to 100xc2x0 C. or so by means of a heat exchanger and then introduced into a collection column, these adducts condense within the heat exchanger or in the pipe connecting the heat exchanger with the collection column, and grow to form tarry matter. (2) Accordingly, if the piping section extending from the outlet of the reactor to the inlet of the collection column is maintained at a temperature which is not lower than 200xc2x0 C., or if the reaction gas is brought into contact with the same collector as used in the collection column prior to the condensation and growth of the adducts contained therein (i.e., immediately after the reaction gas emerges from the outlet of the catalyst bed within the reactor and preferably in the neighborhood of the outlet of the catalyst bed) so as to incorporate the adducts into the collector, the condensation and growth of the adducts and hence the adhesion of tarry matter can be effectively prevented.
The alkanolamine used as the raw material in the present invention is a compound represented by the general formula (1): 
wherein R is hydrogen, methyl or ethyl X is hydroxyl or amino, and Y is amino when X is hydroxyl or hydroxyl when X is amino. Typical examples of this alkanolamine include monoethanolamine, isopropanolamine, 2-amino-1-propanol-1-amino-2-butanol and 2-amino-1-butanol.
The aziridine compound is a compound which is obtained by the dehydration reaction of the above-described alkanolamine and represented by the general formula (2): 
wherein R has the same meaning as defined for the general formula (1). Typical examples of this aziridine compound include ethyleneimine, 2-methylethyleneimine and 2-ethylethyleneimine.
No particular limitation is placed on the process for preparing an aziridine compound by the dehydration reaction of an alkanolamine, and any of commonly known processes may be employed. Specifically, there may be employed, for example, a process comprising a reaction step for feeding a vaporized alkanolamine to a reactor where an aziridine compound is formed by the dehydration reaction of the alkanolamine, a collection step for feeding the reaction gas emerging from the reactor to a collection column where the aziridine compound is collected from the reaction gas, and a purification step for feeding the collector liquid to a distillation column where it is distilled to obtain an aziridine compound product; or a process comprising the same reaction step as described above, a condensation step for cooling the reaction gas emerging from the reactor so as to condense it, and a purification step for feeding the resulting condensate to a distiller where it is distilled to obtain an aziridine compound product. For the details of these processes, reference may be made to the aforementioned patents.
As the reactor, there may be used any of the reactors which are commonly used for the dehydration reaction of alkanolamines. When the dehydration reaction is carried out in the reactor, the temperature is usually in the range of 300 to 500xc2x0 C. and preferably 350 to 450xc2x0 C. The pressure may be atmospheric, reduced or elevated. In the case of reduced pressure, it may be in the range of 13 to 667 hPa (10 to 500 mmHg). The space velocity is usually in the range of 10 to 20,000 hxe2x88x921 and preferably 50 to 5,000 hxe2x88x921 (STP). Under reduced pressure, it is in the range of 50 to 2,000 hxe2x88x921 (STP). No particular limitation is placed on the type of the catalyst used for the dehydration reaction, and there may be used any of various commonly used catalysts including, for example, those described in European Patent Laid-Open Nos. 227461, 228898 and 230776.
As the collection column, there may be used a packed tower, a plate tower, a multitubular tower, a spray tower, a wetted wall tower or a combination thereof. As the collector, there may be used a member suitably selected from among the collectors which are commonly used for the collection of aziridine compounds. Although the same alkanolamine as that used as the raw material or an aqueous solution thereof may be used, it is convenient and economical to utilize the condensate of the reaction product. For example, after the liquid accumulated at the bottom of the collection column is passed through an external heat exchanger to deprive the liquid of sensible heat and heat of condensation, a portion of the liquid may be used as the collector. In this case, some of the raw material may be added thereto.
As the distillation column, there may be used any type of distillation column, such as a packed tower or a plate tower. The distillation process may be a continuous process or a batch process, and may be carried out under any of atmospheric, reduced and elevated pressures. The aziridine compound is discharged from the top of the distillation column and further purified, if necessary, to obtain an aziridine compound product. On the other hand, the bottoms including unreacted alkanolamine and by-products (e.g., produced water, Schiff bases, ketimines and piperazines) are withdrawn from the bottom of the distillation column. These bottoms are usually fed to a recovery column, where the alkanolamine is recovered. For the details of this recovery step, reference may be made to the aforementioned Japanese Patent Laid-Open No. 217659/""92.
As the cooler, a heat exchanger is usually used.
One feature of the present invention is [I] that, when an aziridine compound is prepared from an alkanolamine in the vapor phase according to a process comprising a reaction step, a collection step and a purification step, the adhesion of tarry matter to the piping section extending from the outlet of the reactor to the inlet of the collection column, and to other sections can be prevented (A) by maintaining at least the piping section extending from the outlet of the reactor to the inlet of the collection column at a temperature which is not lower than 200xc2x0 C., or (B) by bringing the reaction gas into contact with a collector prior to its introduction into the collection column, preferably in the neighborhood of the outlet of the catalyst bed within the reactor. Another feature of the present invention is [II] that, when an aziridine compound is prepared according to a process comprising a reaction step, a condensation step and a purification step and requiring no collection step, the adhesion of tarry matter to the piping section extending from the outlet of the reactor to the cooler, the inside of the cooler, and other sections can be prevented (C) by bringing the reaction gas into contact with a collector prior to its introduction into the cooler, preferably in the neighborhood of the outlet of the catalyst bed within the reactor.
In the processes (A) and (B), the term xe2x80x9cpiping sectionxe2x80x9d means not only the pipe connecting the outlet of the reactor with the inlet of the collection column, but also the cooler (e.g., heat exchanger) optionally installed before the collection column.
In the process (A), at least the piping section extending from the outlet of the reactor to the inlet of the collection column is maintained at a temperature which is not lower than 200xc2x0 C. This may be accomplished by providing the piping section with a commonly used heating means (e.g., a heater) and heating the piping section therewith. As used herein, the temperature of the piping section refers to the temperature of the outer wall of the piping section. The piping section is usually maintained at a temperature in the range of 200 to 500xc2x0 C. and preferably 300 to 400xc2x0 C. If the temperature of the piping section is excessively high, the decomposition and combustion of the aziridine compound present in the reaction gas will occur to cause such problems as a reduction in yield.
Moreover, the section extending from the outlet of the catalyst bed within the reactor to the outlet of the reactor may also be heated (for example, by external heating) so as to minimize a reduction in the temperature of the reaction gas. This makes it possible to prevent the adhesion of tarry matter more effectively.
Furthermore, the adhesion of tarry matter can be more effectively prevented by bringing the reaction gas into contact with a collector prior to its introduction into the collection column, preferably in the neighborhood of the outlet of the catalyst bed within the reactor. The collector used for this purpose is usually the same as that used in the collection step.
Since a relatively hot reaction gas is fed to the collection column in the process (A), the degree of collection of the aziridine compound in the collection column is lower as compared with the conventional techniques in which the reaction gas is introduced into the collection column after being condensed or cooled to 100xc2x0 C. or so. Consequently, it is preferable to cool, or cool to condense, the gas discharged from the collection column, introduce the gas into a second collection column, and thereby recover the aziridine compound. In this case, the pipe extending from the collection column to the cooler, the cooler, and the pipe extending from the cooler to the second collection column will not suffer from the adhesion of tarry matter. In addition, as shown in the flow sheet of FIG. 1 which will be given later, it is also possible to recover the aziridine compound by storing the collector liquid from collection column 103 and the condensate from cooler 104 in a storage tank 105 and introducing the gas discharged from storage tank 105 into second collection column 106.
In the process (B), it is preferable to bring the reaction gas into contact with the collector in the neighborhood of the outlet of the catalyst bed within the reactor, because this makes it possible to prevent the adhesion of tarry matter more effectively. The collector used for this purpose is usually the same as that used in the collection step. In the process (B), it is unnecessary to heat the piping section and thereby maintain its temperature as dictated in the process (A).
Also in the process (C), it is preferable to bring the reaction gas into contact with the collector in the neighborhood of the outlet of the catalyst bed within the reactor, because this makes it possible to prevent the adhesion of tarry matter more effectively. The collector used for this purpose is usually the same as that used in the collection step. For further information on the conditions of the reaction step and the like, reference may be made to Japanese Patent Publication No. 55498/""93.